Commentary Magazine

Gravity Waves Observed

Los Alamos National Laboratory

One of the great events in the history of astronomical observation took place last August 17th.

Two neutron stars, 130 million light-years away, spiraled in towards each other closer and closer. As they converged, they produced what is known as a kilonova: an explosion so violent that it distorts space-time itself. This burst of energy produces the gravity waves that were predicted by Einstein in 1915 and finally detected last year (a feat that won the Nobel Prize in physics this year). The New York Times has a great animation showing what happened.

What made this detection different is that we not only observed the gravity waves the event created, we saw the event itself both visually and with radio telescopes, producing a wealth of data that will greatly increase our understanding of the process.

Stars shine by fusing hydrogen into helium in their cores. The sun converts about 600 million tons of hydrogen into 596 million tons of helium every second. The 4 million ton difference is converted into energy according to the world’s most famous equation, E = MC². The energy is measured in ergs (a very small unit of energy), the mass in grams, and C is the speed of light in kilometers per second. So the energy locked in one ton of hydrogen equals 2,000 pounds times 453 grams times 300,000 times 300,000. That’s a lot of ergs. Multiply that by 4 million, and you can see why it’s possible to get a bad sunburn in less than an hour.

When a star such as the sun runs out of hydrogen, it evolves into a white dwarf roughly the size of the Earth but with, very roughly, the mass of the sun—a stellar cinder, slowly cooling down. A teaspoon of a white dwarf would weigh several hundred tons.

But when a star of at least eight solar masses runs out of fuel, the result is a supernova: a titanic explosion that briefly outshines the rest of the galaxy and produces a neutron star (or, if large enough, a black hole). Neutron stars are roughly ten miles across, but, again, have about one solar mass. So a teaspoon of neutron star would weigh about the same as Mt. Everest.

What do kilonovas have to do with our quotidian existence? Well, they are, fortunately, very far away (or, at least, we hope they are: one taking place anywhere in the Milky Way would probably have very nasty consequences for planet Earth), but they’ve been going on since not long after the birth of the universe 13.8 billion years ago.

And the neutron-rich atoms at the higher end of the periodic table are created in these extraordinary explosions. So, if you’re married, take a look at your gold or platinum wedding ring. The metal it’s made of was created at the instant and in the heart of a kilonova.

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